1. Field of the Invention
The present invention relates to an electrodeless discharge lamp.
2. Description of the Related Art
In recent years, from the standpoint of protection of a terrestrial environment, resource saving has been advocated in various industries. An effective measure to save the resources for a lamp is to obtain a longer operating lifetime. An electrodeless discharge lamp has been receiving considerable attention because its operating lifetime is typically several times longer than that of a discharge lamp having an electrode with a filament.
The structure of an electrodeless discharge lamp is different from that of a conventional fluorescent lamp having a filament. For example, an electrodeless discharge lamp includes: an electrodeless bulb filled with a light emitting material; an induction coil for providing electromagnetic energy that excites the light emitting material (discharge gas) in the bulb so as to allow the material to emit light; and a high frequency power supply circuit for supplying high frequency electric power to the induction coil.
The core of the induction coil of the electrodeless discharge lamp includes a magnetic material. If the temperature of the magnetic core of the induction coil increases during a discharging operation of the lamp and exceeds a certain critical temperature (Curie temperature), the magnetic permeability of the magnetic core decreases so that the operation of the discharge lamp is stopped. Thus, the electrodeless discharge lamp should have a structure that decreases the temperature of the induction coil.
The electrodeless discharge lamp has a long operating lifetime, but the lifetime is not unlimited, because the lifetime of electric components used in the lamp is finite. The lifetime of the electric components depends on the temperature. Especially, the lifetime of an electrolytic capacitor used in the power supply circuit of the electrodeless discharge lamp largely varies according to its environmental temperature. As the environmental temperature increases by 10xc2x0 C., the lifetime of the electrolytic capacitor is decreased by half, which is known as the xe2x80x9cArrhenius"" 10-degree rulexe2x80x9d. Thus, the electrodeless discharge lamp should have a structure that decreases the temperature of the electric components.
Under such circumstances, considerable effort has been made to decrease the temperatures of the induction coil and the electric components incorporated in the electrodeless discharge lamp as much as possible.
For example, Japanese Utility Model Publication for Opposition No. 6-6448 discloses a known technique for decreasing the temperature of a magnetic core of an induction coil. According to this technique, a stick-shaped heat conducting member is incorporated in a central portion of a cross-section of the magnetic core along a longitudinal direction of the magnetic core for releasing heat generated in the induction coil. The stick-shaped heat conducting member is connected to a metal jacket, which is a casing of the induction coil, such that the heat conducted through the stick-shaped heat conducting member is radiated from the casing. The metal jacket extends from the lamp so as to radiate the heat.
In the above structure disclosed in Japanese Utility Model Publication for Opposition No. 6-6448, the heat generated in the magnetic core of the induction coil is transmitted to the metal jacket, which is a casing of the induction coil. This technique can decrease the temperature of the induction coil, but on the other hand, the temperature of the electric components included in the power supply circuit provided inside the casing is increased due to the heat transmitted to the casing.
Japanese Utility Model Publication for Opposition No. 6-6448 does not discuss issues concerning heat liberation from the electric components.
According to one aspect of the present invention, an electrodeless discharge lamp includes a bulb filled with discharge gas; an induction coil for generating an electromagnetic field in the bulb; a power supply circuit which includes a plurality of electric components and which supplies electric power to the induction coil; a case for accommodating the plurality of electric components, the case having a surface which faces an external element of the electrodeless discharge lamp; a stick-shaped heat conducting member placed along a winding axis of the induction coil; a first planar heat conducting member placed so as to be perpendicular to the winding axis, the first heat conducting member being thermally connected to the stick-shaped heat conducting member; a second heat conducting member thermally connected to the first heat conducting member; and a third heat conducting member for thermally connecting at least two of the plurality of electric components, wherein the second heat conducting member is thermally connected to the case, and the thermal conductivity of the first heat conducting member is lower than the thermal conductivity of the stick-shaped heat conducting member and the thermal conductivity of the second heat conducting member.
In one embodiment of the present invention, the first heat conducting member is made of an electrically insulative material.
In another embodiment of the present invention, the first heat conducting member is made of a ferrite material.
In still another embodiment of the present invention, the thermal conductivity of the first heat conducting member is between about 0.8 W/mxc2x7K and about 6 W/mxc2x7K inclusive; and the thermal conductivity of the stick-shaped heat conducting member and the second heat conducting member is between about 100 W/mxc2x7K and about 400 W/mxc2x7K inclusive.
According to another aspect of the present invention, an electrodeless discharge lamp includes: a bulb filled with discharge gas; an induction coil for generating an electromagnetic field in the bulb; a power supply circuit which includes a plurality of electric components and which supplies electric power to the induction coil; a case for accommodating the plurality of electric components, the case having a surface which faces an external element of the electrodeless discharge lamp; a stick-shaped heat conducting member placed along a winding axis of the induction coil; a second heat conducting member thermally connected to the stick-shaped heat conducting member at a joint surface; and a third heat conducting member for thermally connecting at least two of the plurality of electric components, wherein the second heat conducting member is thermally connected to the case, and a product of the thermal conductivity of the stick-shaped heat conducting member and the area of a cross-section of the stick-shaped heat conducting member which is perpendicular to a central axis of the stick-shaped heat conducting member is smaller than a product of the thermal conductivity of the second heat conducting member and the area of the joint surface.
In one embodiment of the present invention, the electrodeless discharge lamp further includes a reduction member placed between the induction coil and the second heat conducting member for reducing a thermal influence caused by an electromagnetic field generated by the induction coil on the second heat conducting member.
In another embodiment of the present invention, the reduction member is made of a ferrite material.
According to still another aspect of the present invention, an electrodeless discharge lamp includes: a bulb filled with discharge gas; an induction coil for generating an electromagnetic field in the bulb; a power supply circuit which includes a plurality of electric components and which supplies electric power to the induction coil; a case for accommodating the plurality of electric components, the case having a surface which faces an external element of the electrodeless discharge lamp; a stick-shaped heat conducting member placed along a winding axis of the induction coil; a first planar heat conducting member placed so as to be perpendicular to the winding axis, the first heat conducting member being thermally connected to the stick-shaped heat conducting member; a second heat conducting member thermally connected to the first heat conducting member; a third heat conducting member for thermally connecting at least two of the plurality of electric components; and a printed circuit board on which the plurality of electric components are placed, wherein the second heat conducting member is thermally connected to the case, the third heat conducting member is thermally connected to the case, and there is a space between the printed circuit board and the second heat conducting member.
According to still another aspect of the present invention, an electrodeless discharge lamp includes a bulb filled with discharge gas; an induction coil for generating an electromagnetic field in the bulb; a power supply circuit which includes a plurality of electric components and which supplies electric power to the induction coil; a case for accommodating the plurality of electric components, the case having a surface which faces an external element of the electrodeless discharge lamp; a stick-shaped heat conducting member placed along a winding axis of the induction coil; a first planar heat conducting member placed so as to be perpendicular to the winding axis, the first heat conducting member being thermally connected to the stick-shaped heat conducting member; and a second heat conducting member thermally connected to the first heat conducting member, wherein the second heat conducting member is in contact with the case at a surface contact portion.
In one embodiment of the present invention, the electrodeless discharge lamp further includes a third heat conducting member for thermally connecting at least two of the plurality of electric components with each other.
In another embodiment of the present invention, the second heat conducting member is pressed into the case.
In still another embodiment of the present invention, the second heat conducting member is in contact with the case at the surface contact portion through thermally conductive grease.
In still another embodiment of the present invention, the thermal conductivity of the thermally conductive grease is between about 0.2 W/mxc2x7K and about 6 W/mxc2x7K inclusive.
In still another embodiment of the present invention, the induction coil includes a magnetic core having a hollow portion; and the stick-shaped heat conducting member is fitted in the hollow portion of the magnetic core.
In still another embodiment of the present invention, the stick-shaped heat conducting member is provided so as to envelop the induction coil.
In still another embodiment of the present invention, the case is made of a resin material.
In still another embodiment of the present invention, the plurality of electric components include an electrolytic capacitor; and the electrolytic capacitor is placed so as not to be in contact with the third heat conducting member.
In still another embodiment of the present invention, the case has a base for receiving a commercial electric power which is to be supplied to the plurality of electric components; and the electrolytic capacitor is placed such that at least a portion of the electrolytic capacitor is within the base.
In still another embodiment of the present invention, the thermal conductivity of the third heat conducting member is between about 0.2 W/mxc2x7K and about 4 W/mxc2x7K inclusive.
Thus, the invention described herein makes possible the advantages of providing an electrodeless discharge lamp wherein the temperature of a magnetic core of an induction coil is maintained to be equal to or smaller than its critical temperature while an increase in temperature of electric components is suppressed.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.